1. Field of the Invention
The present invention relates to a sheet stacking apparatus for stacking sheets, and a method of controlling the sheet stacking apparatus.
2. Description of the Related Art
With recent progress of the technology, image forming apparatuses come to form images on sheets at increased speed, and in accordance therewith, sheet stacking apparatuses that stack a large amount of sheets discharged from the image forming apparatuses at the increased speed have also been demanded to stack an even larger amount of sheets with a still higher accuracy.
A technique concerning such a large capacity sheet stacking apparatus (hereinafter referred to as “the stacker”) has been disclosed e.g. in Japanese Patent Laid-Open Publication No. 2006-124052. A stacker disclosed in Japanese Patent Laid-Open Publication No. 2006-124052 proposes a compact stacker which is capable of detecting a fully stacked state of sheets on a tray. This conventional stacker will be described with reference to FIG. 27.
FIG. 27 is a schematic cross-sectional view of the conventional stacker.
A sheet discharged from the image forming apparatus is received by an inlet roller 501, and then a leading end thereof is passed to a gripper 503 by a conveying roller 502. The gripper 503 conveys the sheet while gripping the leading end thereof. After the leading end of the sheet collides against a leading end stopper 504, the gripper 503 let the sheet fall onto a sheet stacking table 505. By repeatedly carrying out this operation, a predetermined number of sheets are stacked on the sheet stacking table 505.
Depending on the case, the stacker is designed such that whenever a sheet is stacked, an alignment process is carried out for aligning ends of the sheets by an alignment plate, not shown, in a direction orthogonal to a sheet-conveying direction, whereby alignment of sheets is improved.
Further, techniques for stacking a large number of sheets have been disclosed in Japanese Patent Laid-Open Publication No. 2002-338126 and Japanese Patent Laid-Open Publication No. HOB-143209.
In the technique disclosed in Japanese Patent Laid-Open Publication No. 2002-338126, two sheet-stacking spaces are formed by dividing a tray by a partition plate movable in a sheet-discharging direction. When sheets to be stacked are small-size sheets e.g. of A4 or B5 size, it is possible to secure a stacking amount of sheets twice as large as that of an undivided tray, by stacking sheets in the respective sheet-stacking spaces thus formed. When one sheet-stacking space is fully loaded, the partition plate is moved for stacking sheets on the other sheet-stacking space. Since the stacking operation can be continued without taking out stacked sheets, it is possible to shorten a time period over which the associated image forming apparatus is made unavailable, thereby making it possible to enhance working efficiency during stacking of a large amount of sheets.
In the technique disclosed in Japanese Patent Laid-Open Publication No. H08-143209, a plurality of trays are arranged in a stacker in a direction orthogonal to a sheet-conveying direction such that the trays can be switched. Sheets are discharged onto one of the trays arranged as above, and when the one is fully loaded, it is switched to another on which sheets can be stacked, whereby it is possible to stack sheets without making the stacker unavailable. This makes it possible to secure a stacking amount of sheets, which is equal to that provided by a plurality of stackers, which makes it possible to downsize the stacker.
In the above-mentioned conventional sheet stacking apparatus, however, when large-size sheets e.g. of A3 or B4 size are stacked, it is necessary to adapt the size of trays to that of the large-size sheets. As a result, when small-size sheets are stacked, a useless space incapable of stacking sheets increases on each tray, whereby it is impossible to make an efficient use of space within the stacker. This makes it impossible to efficiently stack sheets of various sizes while realizing compactness of the stacker.